This invention relates to ceramic coated laminates for printed wiring boards excellent in heat resistance and thermal conductivity, and a process for producing the same.
Heretofore, as printed wiring boards, there have widely been used phenol resin laminates and epoxy resin laminates. But with a recent demand for high density packaging of parts caused by higher performance and miniaturization of electronic devices, there arises a problem how to treat high-density generation of the heat thus generated. Since these conventional organic material substrates are poor in heat dissipation properties due to poor thermal conductivity and also poor in heat resistance, it was difficult to use these organic material substrates for high-density packaging. Thus, as substrates having excellent thermal conductivity, there have been noticed alumina and the like ceramic substrates, metal core substrates obtained by coating a metal plate as a core with an insulating layer. Further, from the viewpoint of heat resistance, there have been developed substrates obtained by using heat resistant resins such as polyimides, polyetherether-ketones, polysulfones, and the like in place of conventional phenol resins and epoxy resins.
But these substrates have many problems. For example, the substrates made of ceramic such as alumina, silicon carbide, beryllia, etc. are excellent in thermal conductivity and heat resistance, but they have defects in that production steps are complicated, the workability is poor, mechanical strength is low, a large size substrate cannot be obtained due to the limitation to the size of substrate, and the like. The metal core substrates obtained by forming woven fabric reinforced resin layers on surfaces of a metal plate core cannot sufficiently use the high thermal conductivity of the metal core and thus are insufficient in heat dissipation properties, since the insulating layer made from a resin having low thermal conductivity contacts with electrical conductive portions which is to be formed into a circuit. Further, since the core material is made of a metal, the formation of through holes is not easy and requires very complicated production steps. As to the heat resistant resin substrates, they are improved in the heat resistance, but good heat dissipation properties cannot be expected due to low thermal conductivity of the resins.
In order to improve these disadvantages mentioned above, there is proposed a process wherein a ceramic is plasma flame sprayed on an ordinary printed circuit substrate to form an insulating layer good in thermal conductivity, and an electrical circuit is formed thereon (Japanese Patent Unexamined Publication No. 60-62186). But when a ceramic is simply flame sprayed on an ordinary printed circuit substrate, the adhesiveness between the ceramic layer and the substrate is not sufficient due to the difference in thermal expansion coefficients of the ceramic layer and the substrate, and the like. Thus, the formation of electrical circuits sufficient in reliability is difficult. Further, when a thicker ceramic layer is formed in order to improve the heat conductivity, there arises a problem in that the ceramic layer is peeled off from the ordinary substrate due to the difference in thermal expansion, and the like.